1. Field of the Invention
The invention herein relates to couplers and attenuators for light transmitting optical fibers. It further relates to a novel method for the use of such couplers and attenuators.
2. Description of the Prior Art
Fiber optic light transmission over any significant distance or in a system with branches invariably requires that the optical fibers be coupled together. In the past this coupling has been accomplished in one of two ways.
Where the fibers are to be joined end-to-end, as in an extended cable, past practice has been to fabricate optically smooth parallel faces on the ends of the adjacent fibers and then butt the fiber ends together in precise alignment. Prior art couplers for this purpose have therefore been structured primarily to maintain the precise alignment of the fibers, since any misalignment substantially reduces the efficiency of the light transmission from the input fibers to the output fibers. See Hecht, Understanding Fiber Optics, Chapter 8 (1987). These types of couplers are rarely entirely satisfactory. Alignment of single fibers end-to-end is difficult and misalignment is common. When there are matching fiber bundles containing pluralities of fibers to be aligned end-to-end, the alignment problems are multiplied, and correct alignment of all of the fiber pairs simultaneous is virtually impossible. In addition, the abutting optical surfaces at the ends of the fibers are rarely precisely parallel, so that when the fibers are butted together the opposing surfaces are usually not in contact over their entire surface areas.
Further, the "butted fiber" couplers are not capable of coupling two bundles of fibers where the fiber counts of the two bundles differ, unless the overall end surface area of the two abutting bundles is approximately equal, such as where the individual fibers in the lower fiber count bundle are of greater diameter than the fibers in the other bundle. However, since the number of fibers in the two bundles are still different, there will be fiber end surfaces in each bundle which are not aligned with fiber end surfaces of the other bundle, thus reducing the effectiveness of the light transmission.
End-to-end abutment is also used for branching coupling, but accurate end surface contact and alignment has been impossible to obtain. Usually the problem has been addressed not by a coupler, but rather by terminating each fiber in a rounded end to distribute light. The light from the terminated upstream fibers is then passed through one or more external lenses, which redirect portions of the light to the receiving ends of the various downstream fibers. Efficiency of transmission is poor, and misalignment of or damage to the external lens system is frequent.
Both of these types of problems are also described in the aforementioned Hecht reference.
In order to overcome these difficulties, a number of coupler systems have been proposed in which the input and output optical fibers are not butted end-to-end but rather overlap each other and are aligned in parallel. Any cladding of the fibers is removed over a portion of the fibers' length where they are adjacent. The fibers are then twisted and/or melted together or forced into contact by shrink-wrapping of tubing around the fibers, so that light is transmitted through the sides of the contacting fibers or through the homogenous melted fiber mass rather than through the ends of the fibers. Typical of such systems are those shown in U.S. Pat. Nos. 4,291,940; 4,632,513; 4,772,085 and 4,923,268.
Such systems have been some improvement in that they permit both end-to-end transmission of light and branching transmission of light, as well as joining of fiber bundles of slightly different fiber counts. However, these systems have also not been particularly satisfactory, since twisting or melting of the fibers is normally accompanied by physical damage or undesirable physical characteristic changes which adversely affect the efficiency of the light transfer. In addition, such systems (particularly those using melting or shrink-wrapping) normally require special equipment for heating the fibers or the shrink-wrap. Such equipment is often not readily available, particularly in the field, and in any case is usually cumbersome and difficult or dangerous to use, particularly when fiber optic cables are to be spliced in cramped or confined quarters.
It would therefore be of considerable interest to have a fiber optic coupling device and a method of using that coupling which would allow fast, secure and effective means for joining two or more optical fibers to produce highly effective light transmission. It would also be advantageous for such coupler to permit the joining of fiber bundles of any fiber count, including bundles of quite different fiber count, regardless of the actual physical diameters of the opposing end surfaces of the fiber optic bundles to be joined.